Coverting mysql keys

Excuse my complete ignorance of SQL.

I am converting a mysql DB to postgres.

Mysql defines keys in the Create table structure like:
create table ohead (custid  varchar(8),       key     orderid (orderid),
.....

To covert this to postgres I guess I need an index, which is done
externally to the create table?

:
create table ohead (custid  varchar(8),       orderid (orderid),
...

CREATE INDEX orderid(??use same name or _idx??) on ohead (orderid);

Again, I am clueless to SQL, how do I utilize the increase in speed? 
Does it happen when I SELECT/JOIN things together? or do I reference the
index in my queries.

Thanks!

-- 
Zot O'Connor

www.ZotConsulting.com
www.WhiteKnightHackers.com